Bolometers are widely used for sensing low radiation of light, generally in the IR band. In most conventional cases, the bolometers are provided in a form of a focal plan array (FPA), wherein the array comprises a plurality of individual sensing elements (hereinafter also referred to as or “pixels”, or “pixel detectors”). A significant advantage of the bolometer type sensors is their reduced weight and power consumption, particularly due to the fact that they do not require cryogenic cooling. In addition, they are generally much less expensive in comparison with cooled focal plan arrays. However, the typical sensitivity of bolometer type sensors is significantly lower than of cooled-type sensors. Moreover, as bolometer type sensors are very sensitive to temperature variation, they require special means for stabilizing the temperature of the array substrate, and for compensating each individual detector for said temperature variations. It should be noted that the case that accommodates the FPA contributes roughly 80% of the IR flux. Thus, it is of vital importance to monitor the case temperature or its radiation.
Vox (Vanadium Oxide) resistors are widely used in typical bolometers, as the Vox has a relatively large TCR (temperature coefficient of resistance), and low 1/f contribution.
Typical bolometer FPAs are required to detect radiation with a resolution in the order of 50° mK of the scenery temperature. The temperature variations at the bolometer detector due to the heat variations within the scenery are in the order of 0.01-0.1° mK. It should be noted that in order to bring the bolometer detector to its operational point, it is required to heat the active resistor of the detector (the resistor which is exposed to the scenery) by a temperature in the order of 4 °. Said necessity to provide a sensitivity and resolution in the range of at least 40 orders less than the heating of the active bolometer resistor generally enforces the use of a differential measurement. The most common and simple circuitry that applies differential measurement is the Wheatstone bridge, and a detector which include a Wheatstone bridge is indeed commonly used in bolometer-type FPAs.
However, even though a Wheatstone bridge which performs a differential measurement is used, the prior art uncooled bolometer-type FPAs are still very sensitive to variations in the ambient temperature, and special compensation circuitry is required for compensating in the FPA pixel level. More particularly, special circuitry is required to compensate for the non-uniformity of the detectors (i.e., to compensate for their different offset and gain), and to further compensate for the non-uniform effect of the change of the ambient temperature on each detector. The said latter non uniformity arises mainly from the fact that each detector has a different relative location with respect to the case walls.
In order to account for the non-uniformity of the FPA pixel detectors, prior art bolometer-type FPA manufacturers, or the users themselves commonly perform pre-measurements which determine the gain and offset of each pixel detector. The measurements are performed for constant, predefined ambient (case) and substrate temperatures. The results of the measurements are provided in two matrices (or look up tables), a gain non-uniformity matrix, and an offset non-uniformity matrix. More particularly, by using said two matrices the gain and offset of each pixel detector are adjusted during the actual use of the FPA. It should be noted that the offset matrix is also updated periodically (for example, every 2-3 minutes) at times when a homogenous image is provided to the FPA. The homogenous image is typically applied by means of a shutter which is closed and masks the FPA from the scenery radiation. Alternatively, in some prior art cases the optics in front of the FPA is brought to a total “out of focus” state, thereby providing to the FPA an essentially homogenous image. During the closure period of the shutter (or the period of “out of focus state”) the FPA of course cannot be used. During said periods the offset matrix is updated. Said procedure of calibration is generally referred to as NUC (Non-Uniformity Correction). It should be noted that each calibration procedure, that takes place, for example, every 2 to 3 minutes, takes several seconds.
Throughout this application whenever reference is made to “closure of shutter”, it should be noted that it is meant to any action that provides a homogeneous image to the FPA, either by means of closure the shutter, by means of performing “out of focus”, etc.
A said, the need to recalibrate the offset of each FPA detector during the closure of a shutter eliminates any active use of the FPA during said recalibration procedure. Said lost time which is dedicated for recalibration may be critical. However, the fact that the prior art lacks means for determining the rate of the non uniformity degradation, and in order to prevent operation with a too high degradation, has enforced the users to apply a relatively high, and constant rate of calibration (as said, for example every 2 to 3 minutes). In many cases this is a too high rate that is used for caution purposes only.
It is therefore an object of the present invention to provide an arrangement that will enable a significant reduction of the calibration rate (which takes place when the shutter is closed), therefore significantly increasing the actual operation time of the FPA.
It is still an object of the invention to provide an arrangement that will enable the performing of non-uniformity calibration only when it is indeed required.
It is still an object of the present invention to provide an indication for the rate of non-uniformity of the array during the actual operation of the array, and in real time.
It is still an object of the present invention to provide said arrangement in a manner which is simple to manufacture.
Other objects and advantages of the present invention will become apparent as the description proceeds.